1. Field of the Invention
The present invention relates to an electrostatic latent image developing toner and an electrophotographic image forming method, and more specifically to an electrostatic latent image developing toner and an electrophotographic image forming method using such electrostatic latent image developing toner, which are capable of producing a toner image excellent in heat-resistant storability and cold fixability, and crease fixability.
2. Description of the Related Art
In the conventional process of forming electrophotographic image directed to form a visible image by electrophotography, a heat roller fixation system has been widely used to fix a toner image composed of an electrostatic latent image developing toner (also simply referred to as “toner”, hereinafter) onto a transfer medium such as paper, in which the transfer medium, having the toner image formed thereon, is allowed to pass between a heat roller and a press roller for fixation. In order to ensure fixability in the heat roller fixation system, or, adherence of the toner onto the transfer medium such as paper, the heat roller is required to have a large heat capacity.
In recent years, from the viewpoint of preventive measures against global warming, increasing demands for energy saving are also directed to the electrophotographic image forming device. Accordingly, for the electrophotographic image forming devices, particularly for those based on the heat roller fixation system, there have been investigated techniques for reducing heat energy necessary to fix the toner image, that is, techniques for lowering fixation temperature. In order to lower the temperature for fixing the toner, it is necessary to lower melting temperature and melt viscosity of a binder resin which configures the toner host particle. However, by lowering the glass transition point or reducing the molecular weight of the binder resin, aiming at lowering the melting temperature or melt viscosity of the binder resin, the toner will cause other problems, such as degradation in heat-resistant storability, or hot offset.
To address such problems, there have been proposed techniques of improving cold fixability of the toner, by using a crystalline resin and an amorphous resin in combination, as the binder resin composing the toner host particle.
From the viewpoint of improving heat-resistant storability of the toner, there has been proposed a technique of using a styrene-acrylic resin and a crystalline polyester resin in combination [see JP-A-2011-145587 (Patent Document 1) or JP-A-2011-149986 (Patent Document 2), for example]. The crystalline polyester resin is advantageous in that it may readily be designed to have low melting point while keeping the glass transition point (Tg) at a high level. Accordingly, by using the styrene-acrylic resin and the crystalline polyester resin in combination, the hot offset, heat-resistant storability and cold fixability may be improved while making effective use of characteristics of the individual resins, and thereby the toner with an excellent fixability may be obtained.
The technique proposed in JP-A-2011-145587 (Patent Document 1) is directed to improve the cold fixability and post-fixation separability by controlling aspect ratio of the crystalline polyester resin particle, which forms a domain (discrete phase) in the binder resin composing the toner host particle, into a specific range. The toner has, however, suffered from that cracks tend to occur at the boundary between the binder resin and the crystalline polyester resin, so that the toner image may readily crack when paper having the toner image fixed thereon is folded. That is, the toner has suffered from poor crease fixability due to poor folding strength.
Meanwhile, the technique proposed in JP-A-2011-149986 (Patent Document 2) is directed to produce a toner host particle with a core-shell structure, with the shell layer thereof composed of a composite resin which contains a crystalline polyester resin and an amorphous styrene-acrylic resin, and with the core thereof composed of an amorphous resin, and successfully forms the shell layer containing the crystalline polyester resin. The crystalline polyester, however, shows only a poor dispersibility in the shell layer due to poor affinity between the crystalline polyester resin and the styrene-acrylic resin, so that the crystalline polyester partially exposes out on the surface of the toner host particle. The toner may, therefore, fail in achieving a sufficient level of heat-resistant storability.    [Patent Document 1] JP-A-2011-145587    [Patent Document 2] JP-A-2011-149986
The present invention was conceived to address the problems and situation described above, and an object of the invention is to provide an electrostatic latent image developing toner which is excellent in heat-resistant storability and cold fixability, and capable of producing an fixed image with a high crease fixability without causing cracks in the toner image when paper having the toner image fixed thereon is folded; and an electrophotographic image forming method using such electrostatic latent image developing toner.